Starcraft II is a marketing nightmare. It is the sequel to a twelve-year-old computer game, a beacon in the forgotten era of Deus Ex and Baldur’s Gate.
SC2 is far from a marketing nightmare. Gamers are eager for a new Starcraft; Blizzard would have to screw up much, much more than online-only play for it to bomb.
Despite this, I agree with both you and the OP - few games have as much "cred" as StarCraft and I think Activision Blizzard have done a poor job marketing it despite huge anticipation. It will probably still succeed though since the game itself is actually quite good.
Note: Sales data from Wikipedia, which has links to the original articles with the results.
Starcraft II will have the same advantage that the Wii did by pulling in many adult gamers that spent hundreds and hundreds of hours playing the original game. I concede the point that the first person shooter Genre has always had a larger user base than the real time strategy genre but I still think that perhaps there could be a lot of demand for the RTS genre that has been marginalized in recent years as things have moved towards MMOs and the more modern FPS games. It will be interesting to see how it plays out and how blizzard is able to leverage their huge (paying) user base to market the this new release.
Maybe this will find success not through the older generation that remembers the original, but instead by giving rise to a new generation of RTS players who never even knew what they were missing in this FPS and MMO dominated world.